Variable duration shock testing machine



July 21, 1953 K. w. JOHNSON 2,645, VARIABLE DURLTION SHOCK TESTINGMACHINE Filed May 31, 1949 INVENTOR. K N/VfT/l W I/IVJOIV Patented July21, 1953 UNITED oFFicE VARIABLE DURATION SHOCK TESTING MACHINE KennethW. Johnson, Dayton, Ohio- Application May 31, 1949, Serial No. 96,383

' 3 Claims. (01. 73-12) (Granted under Title 35, U. s. Code (1952),

see. 266) The invention described herein may be manufactured and used byor for the United states Government for governmental purposes withoutpayment to me of any royalty thereon.

This invention relates to' shock test1ngmachines and has specialreference to'a' shock testing machine in which the duration of the shockis controllable.

Electronic and similar equipment must often be used in situations whereit is subjectedto severe shock, and the shock to which such equ pment issubjected may vary not only in intensity but also in duration.

It is therefore an object of this invention to provide a laboratorymachine which may be set up to simulate shock conditions comparablethose to which the equipment is sub ected in actual practice.

Another object is to provide a machine of this character which may behighly effective for the purpose stated but may be built at a very lowcost in time and material.

Other objects and advantages will be apparent from a consideration ofthe following specnlcation considered with reference to the drawings,

wherein:

Fig. 1 shows the complete test mach ne in front elevation.

Fig. 2 shows the same machine in side elevation. a

Fig. 3 is a horizontal section taken on the line 3-3 of Fig. 1 showing atop plan view of the n box. Fi g. 4 is a horizontal section taken at4-.-4 of Fig. 1 showing a bottom plan view of the work table.

Figs. 5 and 6 show several types of 1mpact members whereby the durationof the shock is controlled.

Like reference characters refer to like parts throughout the severalviews.

In the drawing, a box [0 for holding sand or other granular material isof square plan view and comprises four side plates l2 joined at thecorners of the box and reinforced around the bottom by lengths of angleiron [4 and around the top by lengths of angle iron l6. .Tubular columnsI8 extend upwardly from two diagonally opposite corners of the box andare preferably welded to the side plates. A beam extends across theupper ends of the columns is and 1s rigidly secured thereto.

A work table 22 comprises two metal plates 24 with a plywood layer 26between. Tubular posts 28 are fastened to the table at opposite cornersand extend upwardly alongside the columns 18. A horizontal rail 30 isjoined at its ends to the tops of the posts 28 preferably by Welding.Corner brackets 32 and 34 are secured to the underside of the table 22and the upper side of the rail 30 respectively by bolts 36. Brackets 32and 34 carry. guides for directing free fall of the table along thecolumns I8. 7

H. Carried on the underside of the table 22 by clamping bars 38 are oneor more blocks 4001' one or morewedges 42. The blocks and wedges may bemade of wood, --preferably maple, with the wearing surfaces faced with arelatively thin plateof sheet steel as at 44, 46, 48, 50 and 52, andwith a much thicker steel plate 54 across athe top. The plates 54 extendbeyond the ends of the blocks and wedges. to. provide lips 56 which theclamps 38 engage in fastening the blocks or wedges 42 to the bottom ofthe table 24. Bolts 58 which hold the clamps 38 to the underside of thetable 22 are loosened temporarily while sliding the desired number ofblocks 40 or wedges 42v into place between the .clamps 38. with thelips, 56. under the clamps. The bolts 58 are then tightened to hold theblocks .40 or wedges-42 securely to the underside of the table. Thetable 22, posts 28, rail 30 and the wedges or blocks 40, 42, and theirclamping mechanism may collectively be referred to as the carriage 59.

-A bracket 60 fast on the box 10 supports a hoist 62. The cable 64 ofthe hoist passes over pulleys 66 and 68 to a release I0 which may betripped manually to allow the table 22 to fall freely. Control lever 12is Provided for raising or lowering the table to the desired height. Oneof the columns 18 is calibrated as at I4 toindicate the height to whichthe table hasbeenraised. Bolts 16 are suspended on chains 18 and may beinserted through openings-in columns [8 into openings in posts 28 toforestall accidental dropping of the table.

In order to simplify the calculations incident to the operation of themachine, the carriage" 59 shall at all times carry a load of pounds, 1.e.', the equipment under test plus any dead weight on the table shallequal 100 pounds. Any mounting accessories for holding the equipmentunder test, such as fixtures, racks and the like should beof steel angleiron-since rigidity is very important and any yielding will affect theresults. Since no one fixture may be used for all classes of equipmenttested, no fixture is shown. However, any fixture which is to be usedmust be designed so that its natural period of resonance is not withinthe frequency range of the test.

3 The shock machine itself should be mounted on a suitable concrete baseand leveled before being put into use.

With the machine thus conditioned for use, it is possible to subjectelectronic equipment within the weight range of 1 to 100 pounds toshocks of reproducible magnitude in the laboratory, thereby determiningthe suitability of electronic equipment or any other equipment withinthis weight range for use in service conditions where severe shock maybe encountered.

By assembling a selected number of blocks 40 or wedges 42 on theunderside of the table 22, thus creating various areas it will bepossible to apply a shock of any desired duration within the limits ofthe machine. In this connection it is noted that a wedge shaped areaformed of several wedge shaped blocks 42 may penetrate the said severalinches before stopping the carriage, thus providing a shock ofrelatively long duration, while another area made up of blocks 40 offiat surfaces on the underside will stop the carriage with a veryshallow penetration of the granular material, thus providing a shock ofvery short duration. For consistent results the granular material shouldbe leveled and raked once before each drop with a rake having teethapproximately six inches long spaced about two inches apart.

With the desired acceleration and the time duration known, the unknownfactors, the height ofthe drop necessary, the number of blocks, and the;spacing of the blocks may be calculated.

The equation F=M a (Force=Mass a; acceleration) indicates that a bodycan have uniform ac-' short distance, a body does not acquire sufiicientspeed for the air resistance to affect it appreciably.

Inasmuch as the variations in the net external force acting on a body ofrelatively small surface when falling but a few. feet is very small, itis customary to assume that the force is constant. In the equation F=Mait is therefore assumed that gravity acceleration is constant.

When the table 22, carrying the equipment being tested, is. dropped froma predetermined height, the acceleration a. may be determined by theequation:

wherein,

V1=the initial velocity at the point of contact of the blocks 40 withthe granular material, V2=the final velocity at point of maximumpenetration when the velocity is zero,

t=time which is dependent on the distance of penetration in the granularmaterial necessary to stop the table and is controlled by the selectionof the area on the underside of table 22 made up of blocks 40 or wedges42. V1 may be determined by the equation g=gravity (acceleration) andh=height of free fall of table.

Having thus described an embodiment of my invention, I claim:

1. In a variable duration shock testing machine, a box for holdinggranular material, said box having the top side open, a bed of saidgranular material in said box, columns extending vertically from saidbox, a carriage including a horizontally disposed work table movablevertically along said columns, guide means on said carriage invertically sliding engagement with said columns for maintaining freefall of said carriage along said columns, means to raise said carriageto aselected height connected thereto, means to suspend said carriage atsaid selected height associated therewith, trip means to release saidcarriage for free fall, and a plurality of impact membersmounted on theunderside of said table in side by side relation, the members beingindividually separable therefrom whereby shock duration may be varied.

2. In a variable duration shock testing machine, a sand box open on top,guide columns extending upwardly from said box, a carriage including atable above said sand box for supporting the work to be tested, guidemeans on said carriage in sliding engagement with said columns fordirecting free fall of said carriage with respect to said sand box,means to suspend said carriage at a selected height above said sand box,means associated with the suspending means to trip said means to dropsaid carriage upon the sand in said box, and impact means on theunderside of said work table comprising a plurality of blocks in side byside relation, and clamping means for separably fastening said blocks tothe bottom of said work table whereby shock duration may be varied.

3'. In a variable duration shock testing machine, an open top sand box,a carriage including a table above said sand box for supporting the workto be tested, guide means on said sand box and corresponding guide meanson said carriage for directing free fall of said carriage toward saidbox, means to suspend said carriage at a selected height above said box,means to trip the suspending means to drop said carriage upon the sandin said box, and impact means on the underside of said work table whichconsists of a plurality of wedges, and means for fastening said wedges,points downward to the underside of said work table.

KENNETH W. JOHNSON.

